1. Field of the Invention
The present invention generally relates to a physical random access procedure of a mobile communication terminal, and more particularly to a method for controlling power at which a mobile communication terminal transmits a physical random access channel (PRACH) to a universal mobile telecommunication system (UMTS) terrestrial radio access network (UTRAN) in a UMTS system.
2. Description of the Related Art
A random access channel (RACH) for use in a universal mobile telecommunication system (UMTS) is an uplink (UL) random access channel for accessing a UMTS terrestrial radio access network (UTRAN) corresponding to a mobile communication network. The RACH is used to transmit a radio resource control (RRC) connection message for accessing the UTRAN when a terminal mainly receives a paging signal or attempts a call. Alternatively, the RACH is used to transmit a very small amount of data in a Cell_Forward Access Channel (Cell_FACH) state when the UTRAN is connected and a cell update message or UTRAN registration area (URA) update message is sent.
Because a physical random access procedure using a physical random access channel (PRACH) corresponding to a channel for transmitting the RACH is well known, a detailed description of the physical random access procedure is omitted here. An operation for transmitting a PRACH preamble and transmitting preamble detection information through an acquisition indication channel (AICH) in response to the transmitted PRACH preamble will be briefly described with reference to FIGS. 1 and 2. After transmitting a first preamble 200 of FIG. 2 to a UTRAN 102 through the PRACH in step 110 of FIG. 1, a user equipment (UE) 100 corresponding to a mobile communication terminal waits for the preamble detection information to be received from a Node B of the UTRAN 102. The UTRAN 102 includes the Node B and a radio network controller (RNC).
At this time, the UE 100 sets initial transmission power for transmitting a first PRACH preamble as shown in Equation (1).Preamble_Initial_Power=Primary CPICH TX Power−CPICH—RSCP+UL Interference+Constant Value  (1)
In Equation (1), Preamble_Initial_Power is an initial transmission power value, Primary CPICH TX Power is a transmission power value of a primary common pilot channel (CPICH), CPICH_RSCP is a received signal code power (RSCP) value of the CPICH, and UL Interference is a UL interference value.
Preamble_Initial_Power, Primary CPICH TX Power, CPICH_RSCP, and Constant Value are the values contained in system information transmitted from the UTRAN 102. The CPICH_RSCP value is measured in the UE 100.
After measuring the CPICH_RSCP value and obtaining the initial transmission power value using Equation (1), the UE 100 transmits the first PRACH preamble 200 to the Node B of the UTRAN 102 at the initial transmission power in step 110.
If the UE 100 does not receive preamble detection information from the Node B of the UTRAN 102 through the AICH until a predetermined time elapses, it increases the transmission power, used in a previous preamble, by a preset power ramping step, and then transmits a second preamble 202 at the increased transmission power.
If the UE 100 transmits the second preamble 202 but does not receive preamble detection information from the Node B of the UTRAN 102 through the AICH, it may retransmit a preamble up to a preset maximum number of retransmission times while increasing the transmission power by the power ramping step until the preamble detection information is received from the Node B of the UTRAN 102 through the AICH. That is, the UE 100 transmits a third preamble 204, . . . , an N-th preamble 206.
When the UE 100 transmits an n-th preamble, a transmission power value is computed as shown in Equation (2).P(n)=Preamble_Initial_Power+Power_Ramping_Step×(n−1)  (2)
In Equation (2), P(n) is the transmission power value when the n-th preamble is transmitted, Preamble_Initial_Power is an initial transmission power value obtained by Equation (1), Power_Ramping_Step is a power ramping step value, and n is the number of preamble transmissions. The n value is more than 1 and less than the maximum number of retransmissions. The maximum number of retransmissions is a value received from the UTRAN 102. The power ramping step value is received from the UTRAN 102 and is defined in a range from a minimum of 1 dB to a maximum of 8 dB in the current recommendation.
When the UE 100 does not receive an acknowledge (ACK) from the Node B of the UTRAN 102 or does not receive preamble detection information through the AICH even though preambles have been retransmitted to the maximum number of retransmissions, the UE 100 stops the random access procedure. The case where the UE 100 does not receive an ACK is the case where the UE 100 receives no reply or receives a negative acknowledge (NACK).
When detecting a preamble transmitted from the UE 100, the Node B of the UTRAN 102 transmits preamble detection information to the UE 100 through an AICH 208 in step 112. Upon receiving the preamble detection information, the UE 100 ends the physical random access procedure. When the physical random access procedure is completed, the UE 100 sends a PRACH message 210 to the UTRAN 102 in step 114.
Because a channel environment is better when the mobile communication terminal is located near the node or Node B, a measurement value of CPICH_RSCP is greater than that measured in a worse channel environment. In this case, the mobile communication terminal sets the initial transmission power value to a small value according to Equation (1). A probability in which the preamble detection information, i.e., an ACK, is received is low even when preambles corresponding to the maximum number of retransmissions are transmitted.
For example, the mobile communication terminal receives parameters from a mobile communication network as shown in Table 1 when considering system information received from the Voda network in Germany as of May 2005.
TABLE 1Primary CPICH TX Power29dBmUL Interference−110dBmConstant Value−27dBmPower_Ramping_Step3dBmPreamble_Retrans_Max8
Primary CPICH TX Power, UL Interference, and Constant Value of Table 1 are the same as those of Equation (1), and Power_Ramping_Step of Table 1 is the same as that of Equation (2). In Table 1, Preamble_Retrans_Max is the maximum number of retransmissions.
If CPICH_RSCP is −49 dBm when the mobile communication terminal is located near the node or Node B, an initial transmission power value obtained by Equation (1) is 29 dBm−(−49 dBm)+(−110 dBm)+(−27 dBm)=−59 dBm. The transmission power is −59 dBm+3 dBm×(8−1)=−38 dBm even when a preamble is transmitted 8 times corresponding to the maximum number of retransmissions of Preamble_Retrans_Max while the transmission power is increased by 3 dBm corresponding to Power_Ramping_Step.
According to simulation results, a call failure occurs when the mobile communication terminal does not receive an ACK from the Node B and fails to access the mobile communication network.